From what i can find, the difference is that ergot is parasitic and rare but the fungus growing on darnel is symbiotic and common.
Polish wikipedia says up to 86% of seeds carry it but without source. This paper  says 70% of 'wild populations' of related L. perenne, implies infection in darnel (L. temulentum) is also prevalent, but notes most populations have low frequency of infected individuals and then tries to explain that.
Okay, that makes the article make more sense. Most of the article the author claimed it was quite dangerous, but then they later cited a study which it was up to 10% of a harvest. I didn’t know how to make those make sense together.